1. Field of the Invention
The invention relates to a beam reshaping unit for an illumination system of a microlithographic projection exposure apparatus. Such apparatuses are used to produce large-scale integrated circuits and other micro-structured components.
2. Description of Related Art
In the production of micro-structured components, a plurality of structured layers is applied to a suitable substrate, which can be, for example, a silicon wafer. To structure the layers, these are first covered with a photosensitive resist which is sensitive to light of a particular wavelength range, e.g. light in the deep ultraviolet spectral range (DUV, deep ultraviolet). The wafer coated in this way is then exposed to light in a projection exposure apparatus. Such an apparatus comprises an illumination system and a projection lens. The illumination system illuminates a mask that contains a pattern of structures to be imaged onto the resist with the aid of a projection lens. Since the magnification is generally less than 1, such projection lenses are often referred to as reduction lenses.
After the resist has been developed, the wafer is subjected to an etching or separating process. As a result of this process the top layer is structured according to the pattern on the mask. The remaining resist is then removed from the remaining parts of the layer. This process is repeated until all layers are applied to the wafer.
The efficiency of the projection exposure apparatuses is not only determined by the imaging properties of the projection lens but also by the properties of the illumination system that illuminates the mask. The illumination system contains a light source, e.g. a pulsed laser, and a plurality of optical elements which generate a projection light bundle having the desired properties. Amongst other things, these properties include the angular distribution of the light rays which form the projection light bundle.
Generally at the fore here is the angular distribution of the projection light in the plane into which the mask is introduced during the exposure. If the angular distribution of projection light is specifically adapted to the pattern contained in the mask, this pattern can be imaged with improved image quality onto the wafer covered with the photosensitive resist.
The angular distribution of projection light in the mask plane is often not described as such, but as an intensity distribution in a conjugate pupil plane. This exploits the fact that angles formed between the optical axis and light rays passing a field plane correspond to radial distances at which the respective light rays pass a pupil plane. In a so-called conventional illumination setting, for example, the region illuminated in such a pupil plane is a circular disc which is concentric with the optical axis. At each point in the field plane, light rays therefore impinge with angles of incidence between 0° and a maximum angle determined by the radius of the circular disc.
In so-called non-conventional illumination settings, e.g. ring field, dipole or quadrupole illuminations, the region illuminated in the pupil plane has the shape of a ring which is concentric with the optical axis, or a plurality of separate areas arranged off the optical axis. In these non-conventional illumination settings, only oblique rays illuminate the mask.
To generate an angular distribution of projection light that is optimally adapted to the mask, an optical raster element is generally used, which can be for example a diffractive optical element (DOE) or a microlens array. Further examples of such raster elements are described in U.S. Pat. No. 6,285,443 assigned to the applicant. When changing between different illumination settings, e.g. from a conventional setting to a quadrupole setting, it is generally necessary to change the optical raster element. For fine tuning the angular distribution of illumination, and also to generate annular illumination settings, known illumination systems generally have a zoom axicon objective having an object plane in which the first optical raster element is arranged.
An example of such an illumination system having a zoom axicon objective is described in EP 747 772 A. The zoom axicon objective combines a zoom function for the continuously variable adjustment of the diameter of a light distribution and an axicon function for the radial redistribution of light intensities. The axicon system comprises two mutually axially displaceable axicon elements having mutually facing conical axicon surfaces which can be moved towards one another until they are at zero spacing. By adjusting the zoom axicon objective, it is possible to set different annular intensity distributions in an exit pupil of the zoom axicon objective and, in conventional illumination settings, different degrees of coherence. A second optical raster element, which is located in the exit pupil of the zoom axicon objective, is illuminated with the light distribution, which is generated by the first optical raster segment and the zoom axicon objective, and produces an illuminated field in the mask plane.
Other illumination systems having axicon systems for the radial redistribution of light energy are described, for example, in U.S. Pat. Nos. 5,675,401, 6,377,336 and 6,452,662 assigned to the applicant.
Common to the known axicon systems is the fact that, although the illuminated regions in the pupil plane have the desired geometries, the light intensity distributions within these regions are not satisfactory.